Cant we use a Set or collection as a return type in GAE?

Question

In my code i have used Set<Employees> as a return type to my function addEmp() .

So, im gettin an Compilation error. The Error is:

Compiling module com.employeedepartmentgae.Employeedepartmentgae
   Refreshing module from source
      Validating newly compiled units
         Removing units with errors
            [ERROR] Errors in 'file:/home/wissen18/employeedepartmentgae/src/com/employeedepartmentgae/client/GreetingServiceAsync.java'
               [ERROR] Line 6: The import com.employeedepartmentgae.server.domainobject.Employee cannot be resolved
               [ERROR] Line 18: Employee cannot be resolved to a type
            [ERROR] Errors in 'file:/home/wissen18/employeedepartmentgae/src/com/employeedepartmentgae/client/GreetingService.java'
               [ERROR] Line 6: The import com.employeedepartmentgae.server.domainobject.Employee cannot be resolved
               [ERROR] Line 20: Employee cannot be resolved to a type
            [ERROR] Errors in 'file:/home/wissen18/employeedepartmentgae/src/com/employeedepartmentgae/client/EmployeeWidget.java'
               [ERROR] Line 12: The import com.employeedepartmentgae.server.domainobject.Employee cannot be resolved
               [ERROR] Line 75: The method addEmp(String, String, String, AsyncCallback>) from the type GreetingServiceAsync refers to the missing type Employee
               [ERROR] Line 75: The type new AsyncCallback>(){} must implement the inherited abstract method AsyncCallback>.onSuccess(Set)
               [ERROR] Line 75: Employee cannot be resolved to a type
               [ERROR] Line 94: The method onSuccess(Set) of type new AsyncCallback>(){} must override or implement a supertype method
               [ERROR] Line 94: Employee cannot be resolved to a type
               [ERROR] Line 96: Employee cannot be resolved to a type
               [ERROR] Line 96: Employee cannot be resolved to a type
               [ERROR] Line 98: Employee cannot be resolved to a type
         Removing invalidated units
            [WARN] Compilation unit 'file:/home/wissen18/employeedepartmentgae/src/com/employeedepartmentgae/client/Employeedepartmentgae.java' is removed due to invalid reference(s):
               [WARN] file:/home/wissen18/employeedepartmentgae/src/com/employeedepartmentgae/client/EmployeeWidget.java
            [WARN] Compilation unit 'file:/home/wissen18/employeedepartmentgae/src/com/employeedepartmentgae/client/DepartmentWidget.java' is removed due to invalid reference(s):
               [WARN] file:/home/wissen18/employeedepartmentgae/src/com/employeedepartmentgae/client/GreetingService.java
               [WARN] file:/home/wissen18/employeedepartmentgae/src/com/employeedepartmentgae/client/GreetingServiceAsync.java
   Computing all possible rebind results for 'com.employeedepartmentgae.client.Employeedepartmentgae'
      Rebinding com.employeedepartmentgae.client.Employeedepartmentgae
         Checking rule 
            [ERROR] Unable to find type 'com.employeedepartmentgae.client.Employeedepartmentgae'
               [ERROR] Hint: Previous compiler errors may have made this type unavailable
               [ERROR] Hint: Check the inheritance chain from your module; it may not be inheriting a required module or a module may not be adding its source path entries properly

So please help me.....

Answer 1

This is not a GAE problem, but you do not mention that you are using GWT that has a problem.

From the stack trace it looks like the Employee class uses AsyncCallback which is not implemented properly.

...
[ERROR] Line 75: The type new AsyncCallback>(){} must implement the inherited abstract method AsyncCallback>.onSuccess(Set) ...

My recommendation to you is to look at the GWT docs and also create the HelloWorld app and it will create a sample RPC in it.